Structural transition at the free surface of the nematic liquid crystals MBBA and EBBA

Abstract In this paper we report measurements of the angle θt between the director and the vertical axis at the free surface of the nematic liquid crystals MBBA and EBBA. The basis of our technique is the same as the one reported by Bouchiat and Langevin-Cruchon for measurements on MBBA and PAA. However some important improvements make our technique more suitable and more accurate so that we can measure θt to within ± 1° over the whole range of the nematic phase. We find a new structural transition occurs in MBBA (and EBBA) when the temperature reaches a critical value T 0 close to the clearing temperature of the nematic L.C. Below T 0 the polar angle θt tends to zero as (T0 - T)β, where β ≈ 0.5 is a critical exponent Information about the anchoring energy at the free surface of MBBA is also obtained. The experimental results agree with two recent models of the free surface (Parsons, Mada).